1. Field of the Invention
This invention relates to circuit breakers and their operation and particularly to circuit breakers having a trip mechanism with a bimetal and to an arrangement and method for commutating current out of the bimetal following a trip.
2. Background Information
A type of circuit breaker commonly used in residential and light commercial applications is known as a miniature circuit breaker. Such circuit breakers have a fixed contact and moveable contact carried by a pivoted moveable contact arm of a spring powered operating mechanism. The operating mechanism includes a handle through which the moveable contact arm can be pivoted to manually open and close the contacts. The circuit breaker further includes a thermal-magnetic trip device which responds to overcurrent conditions to automatically actuate the operating mechanism to open the main contacts. A delayed or thermal trip is provided by a bimetal which is held fixed at one end, leaving the other end free to deflect in response to the heat generated by the current passing through the bimetal which is connected in series with the main contacts. A persistent current in excess of a predetermined rated current causes the bimetal to bend sufficiently to unlatch or trip the spring powered operating mechanism, which in turn opens the main contacts. A magnetic or instantaneous trip is provided by a magnetic armature which is attracted by the magnetic field generated by a very high overcurrent such as that associated with a short circuit to also unlatch the operating mechanism and open the main contacts.
While the operating mechanism rapidly opens the main contacts in response to a trip, the current is not immediately interrupted because an arc is struck across the opening main contacts. Thus, even though the main contacts physically separate, current continues to flow through the circuit breaker including the bimetal for up to eight to ten msec before the arc is extinguished. Although such a miniature circuit breaker may have a rated current (maximum continuous current) of, for instance 10 amps, modern electrical distribution systems are capable of delivering many thousands of amperes in response to a short circuit. Even though the magnetic armature responds instantaneously to trip the main contacts open, the continuing high overcurrent supported by the arc rapidly heats the bimetal. Existing miniature circuit breakers can withstand such high currents when operated at a 120 and even 240 volts. However, there are attempts now to extend the application of these miniature circuit breakers to 277 volt installations. It has been found that when interrupting very high currents at these voltages the bimetal deflects so rapidly that it impacts its stop with such force that the bimetal takes a set and loses its calibration. This damage can occur on a single interruption.
In order to reduce the duration of the very high current associated when a short circuit flows through the bimetal and to preclude damage to the bimetal of a miniature circuit breaker, our co-pending application Ser. No. 09/689,497, filed on Oct. 12, 2000, places a bypass conductor in shunt with the bimetal as the moveable contact arm carrying the moveable contact moves to the open position. This bypass conductor is connected at one end to the load terminal to which the bimetal is also connected. The second end of the bypass conductor is positioned so that as the contact arm moves to the open position the arc extends to the free end of the bypass conductor, thereby commutating current to the bypass conductor. Essentially then, the bypass conductor forms a low resistance path in parallel with the current path through the bimetal.
While this arrangement greatly reduces the current flowing through the bimetal until the main arc is extinguished and all current flow ceases, the arc introduces resistance into the parallel current paths so that appreciable current can still flow through the bimetal.
There is a need therefore for an improved circuit breaker with a bimetal in the trip mechanism which can withstand short circuit currents repeatedly without damage.
There is a further need for such an improved circuit breaker and method which can successfully commutate virtually all of the current out of the bimetal during interruption of a very large current such as those associated with a short circuit.
These needs and others are satisfied by the invention which is directed to a circuit breaker and a method in which virtually all of the current is commutated out of the bimetal during interruption of very large overcurrents such as those associated with a short circuit. More particularly, the invention is directed to a circuit breaker in which the bimetal of the trip mechanism electrically contacts a bypass conductor shunting the bimetal as the bimetal deflects in response to the very large overcurrent.
Specifically, the invention is directed to a circuit breaker comprising a pair of main contacts including a fixed contact and a moveable contact. The circuit breaker includes a load terminal and a line terminal to which the fixed contact is connected. The circuit breaker further includes an operating mechanism for opening the main contacts when tripped, and a trip mechanism which includes a bimetal having a fixed end electrically connected to the load terminal and a free end electrically connected to the moveable contact. The bimetal is deflected in response to the very large overcurrent through the main contacts to trip the operating mechanism and thereby open the separable contacts. A bimetal bypass comprising a bypass conductor is connected to the load terminal and is positioned to commutate current passing through the bimetal to the bypass conductor through deflection of the bimetal in response to an overcurrent sufficient to trip the operating mechanism. The free end of the bimetal and the bypass conductor can have secondary contacts which engage to electrically connect the bimetal to the bypass conductor to provide better wear characteristics. Preferably, the bypass conductor is a flat conductive strap with an electrical resistance which is substantially less than that of the bimetal.
The operating mechanism includes a moveable contact arm to which the moveable contact is secured, and a flexible shunt electrically connecting the moveable contact arm to the free end of the bimetal. The bypass conductor can be extended toward the moveable contact arm and positioned to commutate at least some of the current from the moveable contact arm to the bypass conductor. The arrangement is such that current is commutated from the moveable contact arm to the bypass conductor and then the bimetal electrically contacts the bypass conductor to commutate any remaining current through the bimetal to the shunt provided by the bypass conductor. Typically, the trip mechanism also includes a magnetic armature which trips the operating mechanism in response to the very large overcurrent, which brings the moveable contact arm in proximity with the extended bypass conductor and thereby initially commutate current into the bypass conductor.
The invention also embraces the method of commutating current out of the bimetal connected at a free end to the main contacts and at a fixed end to the load terminal in a circuit breaker by connecting a bypass conductor to the load terminal and positioning the bypass conductor to be electrically connected to the free end of the bimetal as the bimetal deflects in response to a very high overcurrent. The method further includes extending the bypass conductor to be adjacent the moveable contact of the main contacts as the main contacts open to commutate at least part of the current from the moveable contact arm to the bypass conductor. The bypass conductor is arranged such that the current is commutated to the bypass conductor from the moveable contact arm before the free end of the bimetal is electrically connected to the bypass conductor.